When conducting the calibration of a pressure sensor, it is important to make sure there are no leaks in the connection between the sensor in the calibrator and the sensor being calibrated. This is important because in a closed, steady-state system, a leak will cause a reduction in pressure and possibly a gradient in pressure across the system.
It may sound obvious, but a leak is what happens when media escapes or enters a system, usually as the result of a pressure gradient from high to low. There are situations that may appear to be a leak but are actually due to other factors. These become apparent when examining the relationship between volume, temperature, and the number of the molecules within a system as expressed in the ideal gas law:
PV=nRT
where P, V and T are pressure, volume and temperature; n is the amount of the substance (number of molecules); and R is the ideal gas constant.
The ideal gas law shows us that pressure multiplied by volume is always equivalent to the number of molecules multiplied by the ideal gas constant and temperature. Therefore, with V and T constant, any decrease in the number of molecules (n↓) will cause a decrease in the pressure (P↓), indicating a leak out of the system. In a system under vacuum, an increase in the number of molecules (n↑) will cause an increase in pressure (P↑), indicating a leak into the system. These are true leaks and should be corrected to maintain a stable system.
To complicate the situation, changes of other variables in the ideal gas law will give the appearance of a leak without any true leak being present. For example, with an increase in volume (V↑) or a decrease in temperature (T↓), pressure will decrease, which looks like a leak. In addition, with an influx of more gas (n↑) or if there is a quick decrease in volume (V↓), the gas will heat up and the pressure will rise. On the other hand, when there is a quick reduction in the amount of gas(n↓) or a quick increase in volume (V↑), the gas will cool down and the pressure will drop.
After a system goes through one of these changes, the system temperature will gradually reach equilibrium with the surrounding environment, which will also change the pressure accordingly. These pressure changes are often — and incorrectly — observed as a leak in the system. But wait... there’s more! If the temperature of the environment changes and is different than the internal gas temperature of the system, the system temperature will tend to equilibrate with the environment. This change in temperature will affect the pressure in a similar way, per the ideal gas law. For this reason, when conducting a calibration, it is important to allow the calibrator and the device under test (DUT) to warm up so there is not a temperature gradient between the environment and the system.
To complicate this even more (and beyond the scope of this post), at low absolute and gauge pressures, the system pressure can be more sensitive and susceptible to system geometry and temperature changes.
Leaks occur when there is a breach in a closed system. The breach might occur at fittings used to connect tubing to the calibrator and the DUT. It could also occur within the calibrator, the DUT or the connecting tubing. Pressure controlling calibrators have manifolds, regulators, check valves and connections to their internal transducers. All of these have the potential to leak. DUTs come in a variety of configurations but all have connections that hold the device in place where there is a potential for leakage. Leaks are due to improper fastening or degradation of the seal due to wear, vibration or stress.
For systems that contain a calibrator, manifolds or tubing, and a DUT, each section of the system can be isolated to check for leaks in each area. It can be said the purpose of a controller/calibrator is to introduce a leak or supply pressure to a system and balance it at a setpoint. From this point of view, the DUT can never truly be leak-free while the controller is in control mode.
A Mensor pressure calibrating controller can be controlled to a pressure above atmospheric and placed in measure mode. This isolates all the gas contained in the system from the pressure regulator and creates a closed system between the pressure transducer in the controller and the DUT. With the controller and environment at an equivalent temperature, the pressure can be monitored to see if it changes. If it is seen to decrease in a continuous way, then you can be certain there is a leak somewhere in the system. If the pressure initially decreases but then stabilizes, there is probably not a leak. The initial decrease is due to the cooling of the internal gas that was heated because of the pressurization. This is a tell-tale sign of a pressure change caused by a temperature variation. To monitor this rate of change, many controllers display the pressure rate of change value in their display for easy readout and monitoring.
To isolate a leak further, vent the system and disconnect the tubing at the measure/control port of the controller and plug the port. Pressurize the system again and observe the pressure in measure mode. If the pressure becomes stable, the leak must be in the portion of the system external to the controller.
If it is determined that the leak is external to the controller in the process connections, manifold or tubing, there are a number of ways to correct these issues. Use a leak detector solution made with deionized water and a surfactant (soapy water), and with the system under pressure, apply the solution to suspect areas. If there is a leak, bubbles will form and grow, indicating a breach. To remediate the discovered leak, tighten or replace tubing and/or fittings and test the system again.
Be sure the pressure fittings, tubing and manifold are adequately rated for the pressure range and media you are using. For fittings or connections that require O-rings, inspect them for metal shavings or nicks. It is good practice to use vacuum grease to lubricate the connection and aid in sealing if the O-ring connection ports are frequently adjusted or changed.
Pressure leaks can be frustrating and may take some time to isolate and resolve. The measures described above should assist you in finding any leaks in your system.
If the leak is determined to be internal to the controller, contact Mensor Customer service and they may be able to troubleshoot over the phone or direct you to return the product to Mensor for repair.
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